Conventionally, as photopolymerizing adhesives ultraviolet light(UV)-polymerizing ones were used, which were obtained by adding a UV polymerization initiator that generates free radicals by absorbing UV, such as benzoinalkylether, 1-cyclohexylphenylketone, to a polyfunctional radical polymerizing monomer (e.g., 1, 6hexanedioldi(meth)acrylate, bisphenol-A-epoxy modified di(meth)acrylate), or a monofunctional radical polymerizing monomer (e.g., N-vinylpyrrolidone hydroxyethyl(meth)acrylate).
The UV-polymerizing adhesives are usually one-liquid type, and show a long pot life and such a practical adhesive property as polymerizability by irradiation of UV for a short time, e.g., a few minutes. Accordingly, they are excellent in handling and in adaptation to a prompt adhering operation so that they have been widely used in replace of adhesives such as epoxy resin adhesives which require mixing a main agent with a curing one or heating at an adhesion step.
Lately, however, fast and simple adhesion as obtainable by conventional photopolymerizing adhesives has been increasingly required for opaque materials and UV-absorbing materials such as colored glass, ceramics, polycarbonates and polyvinyl chloride.
Furthermore, it has been pointed out that UV so far used for polymerization of conventional photopolymerizing adhesives has problems. One problem is that UV has a strong chemical action against a human body and causes danger at work. The other problem is that UV ray apparatus must have facilities such as a specialized power supply and exhausting duct. This will require much cost and make the scale of the facilities bigger. Thus, photopolymerizing adhesives have been increasingly expected which can be polymerized by a safer and simpler light source.
Recently, a new type visible light-polymerizing adhesive, which polymerizes by using visible light, has been used as a photopolymerizing adhesive which meets the above purposes.
As this adhesive, for example, an adhesive comprising at least one ketone selected from fluorenone and diketone and at least one organic peroxide, as initiators for visible light polymerization, is disclosed in JP-A-2/110178. In addition, an adhesive comprising, as a visible light-polymeriztation initiator, camphorquinone or benzil is disclosed in JP-A-2/252775.
Examples of other adhesives of a visible light-polymerization type are those containing, instead of a photopolymerization initiator for conventional UV-polymerizing adhesives, an initiator showing absorbance in visible light range (such as thioxanthone derivatives, e.g., anthraquinone, 3-ketocoumanine, 2-chlorothioxanthone, and isopropylthioxanthone; accridone derivatives, e.g., 10-butyl-2-chloroacridone; or thiopyrylium salt derivatives), or those containing an organic dye such as methylene blue.
Normally, to such visible light polymerizing adhesives is added a tertiary amine (e.g., tri-n-butylamine, dimethylaminoethylmethacrylate, N, N-dimethyl-p-toluidine or N, N-dimethyl-p-benzoate) or an organic peroxide (e.g., benzoyl peroxide), for the purpose of increasing reactivity of a photopolymerization initiator.
These visible light-polymerizing adhesives make adhesion of UV-opaque materials by photopolymerization possible, which was so far difficult. Besides, the visible light-polymerizing adhesives have been increasingly used for, e.g., adhesion of dental resins in the mouth in dental fields, because they allow to use a highly safe visible light-source, such as a halogen lamp, as a light source for polymerization.
To the visible light-polymerizing adhesives may be added organic dyes or pigments so as to provide an adhesive layer with a desired color tone after adhesion and polymerization, which is difficult by using UV-polymerizing adhesives.
However, most of conventional visible light-polymerizing adhesives often turn into yellow or brown color after polymerization due to remaining initiators, or turn into yellow, brown or red color due to an unfavorable side reaction during photopolymerization, not being able to yield a transparent adhesive product.
Furthermore, conventional visible light-polymerizing adhesives have a defect that an initially colorless and transparent color tone after photopolymerization will turn into yellow or brown color by exposure to light containing UV, such as sunlight, for a short time, i.e. a defect of low light-resistance.
An attempt has been made to prevent the above discoloration or coloring of the visible light-polymerizing adhesives after photopolymerization, by reducing an amount of the used initiator or omitting the use of tertiary amines together with the photopolymerization initiator. In this case, however, a new problem occurs in that photopolymerizing activity of the visible light-polymerizing adhesives will be too low to obtain a practical photopolymerizing speed. Thus, this is not a final solution.
Accordingly, conventional visible light polymerizing adhesives have not been applied to adhesion of optical parts such as lens and prism, which require a high transparency after adhesion; to adhesion of window glass and objects for out-door display that requires high light-resistance; or to adhering and building up of glass handicrafts that require an excellent aesthetic property.
Furthermore, in case that a conventional visible light-polymerizing adhesive is mixed with a dye and pigment, color tone will be dark and dull after photopolymerization and it will be difficult to make a colored adhesive product showing an excellent aesthetic feature, because of coloring after photopolymerization and fade-out of colorless transparency by sun light.
This invention has been accomplished with an aim to solve the above disadvantages of conventional visible light-polymerizing adhesives.
Thus, an object of the invention is to provide a visible light-polymerizing adhesive having such an excellent photopolymerizing activity as photopolymerizability for a short time by irradiation of visible light; excellent transparency of exhibiting colorless transparency after photopolymerization; and excellent light-resistance of causing no color change or deterioration after exposure to light containing UV such as sun light.